1. Field of the Invention
The present invention relates to the reclamation of used foundry sand and, more particularly, to a method and apparatus for generating improved removal and combustion of binder materials from used sand during reclamation processing.
2. Description of the Related Art
With the advent of stricter waste disposal standards in light of heightened environmental concerns, most foundries are currently faced with severe waste sand disposal problems. More specifically, many solid waste disposal sites have reached capacity while the cost of shipping waste (i.e. used) sand to operating disposal sites has grown substantially. Likewise, on-sight disposal is generally not a viable alternative in view of the promulgation of environmental protection regulations and because most foundries lack adequate space. To this end, reclamation of used sand, which would otherwise have to be discarded, permits a foundry to operate more efficiently and cost effectively. Reclamation of sand for reuse in certain foundry operations, however, is often complicated by the presence of a non-regenerative or non-reusable carbonaceous coating formed on the used sand which must be removed before the sand may be reused. For example, many foundry operations use synthetic resin binders which are added to the sand prior to forming cores and molds. These binders form a coating on the individual sand grains to bind them together with sufficient strength to resist pressure from the molten metal until it solidifies. As is known to those skilled in the art, these coatings should be eliminated before the sand can be effectively reused in foundries for mold and core making. Other foundries use binderless sand for mold making in conjunction with the "lost foam" evaporative casting method. During the molten metal pouring cycle, the polystyrene plastic patterns vaporize with a portion of these vapors condensing on contact with the sand particles making the sand tacky after repeated use. Again, the used sand has to be either discarded or the tacky coating must be removed.
In order to reuse the sand, it is necessary for the sand to be thermally treated to "burn-off" or "incinerate" any carbonaceous coating remaining on the sand grains which was not eliminated during the casting operation. The terms "burn-off" and "incinerate" refer to the normal combustion process and to the process of removing non-combustible constituents through evaporation. Thermal sand reclamation processing must consistently remove a specified minimum percentage of the binder material or coating before the sand can be effectively reused.
Several thermal sand reclamation systems functioning to "burn-off" coatings from the sand grains are known in the art. One type involves metering a generally horizontal bed ("fluidized bed") of sand through a furnace and exposing the sand bed to a high temperature, oxygen-rich combustion environment to burn off the carbonaceous coating. The horizontal sand bed is agitated ("fluidized") by a turbulent air updraft to provide increased exposure of the sand grains to the combustion environment. Therefore, energy in the form of heated and sufficiently pressurized air is required to turbulently fluidize the sand bed. Likewise, it is known that most prior art systems require relatively long sand retention times in the combustion zone to achieve adequate combustion of the carbonaceous coating. Extended sand retention time results in a relatively low system efficiency. A second type of sand reclamation system includes a stacked columnar fluidized bed wherein a vertical column of sand is maintained and heated along its length to burn-off the carbonaceous coating. A third type of reclamation system includes a horizontal rotary kiln (calciner) having a refractory lined cylindrical vessel which is associated with a burner unit for heating the kiln to a temperature of up to about 2,000.degree. F. The sand is tumbled through the kiln because of the rotary motion to expose the sand to the combustion environment.
The sand reclamation system identified in U.S. Pat. No. 3,686,034, having a common inventor with the present invention, discloses a gravity flow, thermal reclamation process which is substantially free flowing and which provides a dispersion of sand particles as they pass downwardly through a vertical furnace. The gravity fed sand passes sequentially through a series of transversely displaced sand dispersion diffusers (grates) mounted within the open zones of the furnace. The orientation of the free falling sand particles is continuously varied along the length of the furnace between the vertical planes of adjacent diffusers such that substantially the entire surface of each sand particle is exposed to the high temperature oxygen-rich incinerating combustion environment. As such, rapid removal and subsequent combustion of particulate coatings is achieved.
The "prior art" thermal reclamation systems herebefore described are adapted to remove a substantial portion of the binder material coatings deposited on the sand grains. However, the present invention is an improvement over "prior art" systems in that residual coatings remaining on the sand grains can be thermally removed. In this manner, the reclaimed sand is of a higher quality and can be more effectively reused in foundry operations.